Mar 29th, 10, 05:14 PM
Working over a set of #442 SBC heads for learning experience and I am looking for some advice while setting up my springs. 981-16, email@example.com seat load. Should I be shimming all valves with a valve spring micrometer to 1.700 as a base, then using a spring pressure tester such as Moroso 62390 and trying to achieve a 105lbs seat pressure? And if needed I can add or remove shims to get as close to 105lbs as possible?
I just need a little break down of the proper way to install valve springs. The heads are at the machine shop now getting tanked, maged, milled, seats cut, etc.
Mar 29th, 10, 05:43 PM
I dont think that the "on head" valve spring pressure testers are really all that accurate. There are too many variables as to being able to tell exactly when the valve starts to open to get the correct pressure. Those tools are great for comparing springs after a bunch of runs in a drag car, but otherwise you need a "real" spring pressure tester to properly test springs. Have your machine shop check them at 1.700 with the retainer you are going to use and let tell you what they read. Or just set them up at 1.700 and dont worry about it. They are going to change the first time they get warmed up and cool back down anyway. I am sure that Comp has all that in mind when they give a recommended installed hgt.
Just my opinion,
Mar 29th, 10, 05:54 PM
Cheers Bill that's all I was looking to hear.
Ill shim them all up to 1.700, set it aaaand forget it!
Mar 29th, 10, 06:15 PM
I'm guessing 981-16 is a Comp Cams Spring and purchased NEW as the recommended Springs for your Comp Cams Cam with these Specs:
981 1.254 .880 Yes 105 @ 1.700 273 @ 1.250 1.150 370
The recommended Install height is 1.7"
From this information it is expected that you install the spring using shims if necessary at 1.7" (Install Height) to have a Valve Seat Pressure of 105 Lbs that is with Valve Closed.
Note the Spring Rate is 370 Lbs per Inch.
Say the Lift of your Valve Open is .460" using 1.5 Ratio Rockers.
Considering the Spring Rate is 1" @ 370lbs
.460" of lift will have .460"/1" x 370lbs = 170.2 lbs
The Total Pressure when the Valve is Open will be 105lbs + 170.2 lbs = 275.2 lbs and Referred to as either Open or Nose Pressure.
NEW Springs have been tested to be accurate within a certain degree (+ or - ft/lbs) of accuracy and really do not require to be tested when installed at the recommended Height. Used Springs will vary and should be tested at the installed Height such that 1.7" may vary with each spring to achieve the Cam Mfgr's recommended 105Lbs Seat Pressure.
When you are playing with extreme high lift cams, shimming to achieve the recommended Pressures can lead to Coil Bind.